The invention relates to a paste extruder for making hollow articles (profiles), for example, plastic pipes or tubes, particularly of polytetrafluorethylene (PTFE). The extruder includes a machine stand to which are attached an extrusion cylinder having an extrusion mouthpiece to receive a preform composed of the plastic to be extruded; and an extrusion drive for an extrusion piston that can be driven into the extrusion cylinder and thus effects the extrusion of the plastic material. The extruder further includes a mandrel pulling rod that is axially displaceably mounted within the extrusion piston and is movable by a mandrel positioning drive for positioning a mandrel that is fixed to the rod end and cooperates with the extrusion mouthpiece to form the hollow article.
Because of their structural length, such paste extruders require a significant amount of space. A substantial structural length for the paste extruder is needed, because the preforms may have a length of up to 2 m, and the stroke of the extrusion piston must correspond to this length. If additionally the extrusion cylinder is to be charged in the advantageous manner of a breechloader, the extrusion piston must be pulled out of the extrusion cylinder to such an extent during the charging process that sufficient space for the introduction of the preform exists between its pressure face and the extrusion cylinder. This space must generally be significantly larger than the length of the preform because in preferred configurations of such extruders the mandrel shaping the interior profile of the article projects beyond the pressure face of the extrusion piston.
Because of the resulting structural lengths of such extruders, they are frequently designed for a vertical extrusion direction. This, however, requires a correspondingly tall operating space which is in part inconsistent with the multi-purpose use of such extruders.
Such extruders require considerable pressures of up to, for example, 1000 bar to drive the extrusion piston. The drives for the extrusion piston must be dimensioned correspondingly large. It is thus customary to have several cooperating drive assemblies act jointly on the extrusion piston. Furthermore, a drive assembly required to axially position the mandrel must be added to the drive assemblies. Thus, the prior art paste extruders of the above-mentioned type, in addition to the significant great structural length, also have a considerable structural width and height.